1. Field of the Invention
This invention relates to spin coating of a liquid material over a substrate and, more particularly, to a spin coating method and apparatus for minimizing film thickness variations caused by turbulent flows produced with rotation of the substrate.
2. Description of the Prior Art
The accuracy required for pattern forming techniques has been increased to meet recent demands for higher density semiconductor devices. In general, the required accuracy is .+-.10% of the design rule. For example, an accuracy (pattern line width uniformity) of .+-.0.035 .mu.m is required for micro LSI having a design rule of 0.35 .mu.m. The factors which may be considered to degrade the pattern forming accuracy include variations in resist film thickness, resist development rate and resist material characteristics made during the photolithography processes, optical or mechanical variations in the exposure unit, variations in photo mask line width, variations in substrate reflection factor, variations in oxide film thickness, etc. In view of these factors causing line width variations, the variance width (Range), which indicates the resist film thickness uniformity, is required to be less than 5 nm (50 .ANG.) for micro LSI having a design-rule of 0.35 .mu.m.
Furthermore, it is required to improve the productivity by using wafers having a larger diameter. The diameter of the wafers used normally in producing micro LSI is as large as 200 mm (8 inches). However, it becomes more difficult to assure the uniformity of the resist film as the wafer has a larger diameter. This is described further in connection with FIGS. 1A to 1C. A wafer 1 is retained in a horizontal position by means of a wafer chuck 2. A resist is discharged from a resist nozzle 3 onto the wafer 1, as shown in FIG. 1A. Then, the wafer 1 is rotated, as shown in FIG. 1B, to form a resist film thereon, as shown in FIG. 1C. The wafer 1 is rotated normally at a speed ranging from 3000 rpm to 6000 rpm. The thickness of the resist film is dependent on the speed and time of rotation of the wafer 1. The speed of rotation of the wafer 1 has an effect on the uniformity of the resist film thickness. The uniformity of the resist film thickness increases with increasing speed of rotation of the wafer 1. Assuming now that the wafer has a diameter of 125 mm (5 inches), the produced resist film will have a sufficient thickness uniformity if the wafer 1 is rotated at a speed higher than 4000 rpm during the resist spin coating process.
One serious problem associated with the conventional spin coating method is the tendency of turbulent air flows to occur near the resist film coated on the rotating wafer 1. The turbulent air flows cause partial variations in the quantity of evaporation of the resist to degrade the uniformity of the resist film thickness. This tendency increases as the wafer rotation speed increases and, thus, the wafer circumferential speed increases. For example, it was said that variations in the resist film thickness occur near the periphery of the water 1 due to turbulent air flows when a wafer 1 having a diameter of 200 mm (8 inches) is rotated at a speed higher than 4000 rpm. According to the result of the tests conducted by the inventor, it is ensured that a good resist film thickness uniformity can be obtained when the wafer 1 is rotated at a speed less than 3500 rpm. When the wafer 1 is rotated at a speed of 4000 rpm, the resist film thickness 100 varies in a range of 100 nm (1000 .ANG.) in a zone 101 between the periphery of the wafer and 7 mm inside the periphery of the wafer, as shown in FIGS. 2A and 2B. In this case, the Reynolds number is 2.35.times.10.sup.5. The resist coating cannot be made with the Reynolds number exceeding 2.35.times.10.sup.5. As can be seen from FIG. 3, the speed of rotation of the wafer cannot be increased over 3500 rpm for 8 inch wafers. The upper limit of the wafer rotation speed decreases as the diameter of the wafer increases. For example, the maximum possible wafer rotation speed is 2400 rpm for 10 inch wafers and 1500 rpm for 12 inch wafers. It may be considered to achieve a good resist film thickness uniformity at a low speed of rotation of the wafer by decreasing the resist viscosity. However, this is sensitive to ambient temperature and humidity. Accordingly, the need has been recognized for a practical approach which can form a resist film having a sufficient thickness uniformity over a wafer having a large diameter.